1. Field of the Invention
This invention relates to a process for the production of thermoplastics or thermosets containing amide groups by catalytic reaction of carboxylic acids and polyfunctional isocyanates with formation of CO.sub.2, to the plastics obtainable by this process and to their use.
2. Discussion of Related Art
It is generally known that carboxy groups give off carbon dioxide when reacted with isocyanates and can thus contribute towards the blowing reaction in PUR plastics.
Thus, O. Bayer (see "Angewandte Chemie" 59, 1947, pages 257-288) describes on page 267 a foam of a) a polyester of a dicarboxylic acid and a trihydric alcohol containing both free hydroxyl groups and also carboxyl groups with b) a diisocyanate. 50 to 300 kg/m.sup.3 foams were obtained (trade name MOLTOPREN). In the case of the aromatic isocyanates used on an industrial scale, this NCO/COOH reaction is very complicated and is unsuitable for industrial-scale blowing reactions, i.e. for obtaining relatively low densities. In addition, the polyurethanes obtained in this way are often strongly colored.
An improvement was achieved by using formic acid as blowing agent, as described in DE 32 23 567. According to this document, polyurethane foams are produced from carboxylic acids, polyols, diisocyanates, tertiary amines and water. Formic acid is used as the carboxylic acid (see Examples). It is not absolutely essential to use water as blowing agent (see page 6, lines 11 to 27). In addition to organometallic compounds, the catalysts mentioned also include tertiary amines, such as dimethyl benzyl amine, tetramethyl ethylene diamine, triethylene diamine, hexamethylene tetramine and dimethyl cyclohexyl amine. The temperatures at which the reaction takes place are not specifically mentioned.
The disadvantage of this process lies in the evolution of CO (inflammable, toxic) in addition to CO.sub.2 as blowing gas. As in the case of water-blown foams, the formic acid has to be added to the polyol/isocyanate mixture immediately before foaming. In addition, polar formic acid--in exactly the same way as water--is incompatible with most other PUR raw materials.
DE 30 41 589 describes mixed carboxylic acid/carbamic acid anhydrides as blowing agents for the production of foam plastics. They are obtained from aliphatic and/or cycloaliphatic isocyanates by reaction with weak mono- or polycarboxylic acids. The anhydrides must have a melting point of more than 40.degree. C. The evolution of CO.sub.2 requires temperatures of more than 100.degree. C. although in some cases the elimination of CO.sub.2 actually begins at around 60.degree. to 70.degree. C. No catalysts are required for the actual blowing reaction. However, it is possible to use catalysts, for example organometallic compounds, Mannich bases and also tertiary amines, for example n-methyl morpholine, 1,2-diazabicyclo-(2,2,2)-octane, bis-(dimethylaminoalkyl)-piperazine and 1,2-dimethyl imidazole, organometallic compounds (for example organotin compounds) being preferred. Semirigid to rigid PUR foams having a density of at least 128 kg/m.sup.3 are obtained. Apart from the high starting temperature, which makes the blowing agent extremely difficult to use, and the high density of the foams, the process in question is attended by the following disadvantages (see DE 38 40 817, page 2):
"The mixed anhydrides on the one hand should be stable in storage at temperatures of up to about 60.degree. C., even in solution, and on the other hand should develop their blowing effect at temperatures as low as about 80.degree. C. with elimination of carbon dioxide. Accordingly, there are very narrow limits to the temperature at which the carbon dioxide is eliminated. Only aliphatic isocyanates can be used for the production of the mixed anhydrides. By contrast, the aromatic polyisocyanates typically used as polyisocyanate component are unsuitable for the production of the special blowing agents.
To carry out the process, the mixed anhydrides first have to be prepared in a separate reaction and isolated and, finally, have to be carefully mixed with the polyol mixture. These are additional process steps which add to the cost of using these compounds and make their use appear complicated. Ready-to-use polyols containing the blowing agents mentioned are difficult to store and transport safely because the risk of a dangerous buildup of pressure cannot be ruled out in the event of overheating which can occassionally occur despite careful handling."
DE 26 07 999 describes foams which are obtained by reaction of hydroxyfunctional organic compounds with an excess of polyisocyanates in the presence of isocyanate trimerization catalysts, blowing agents and, optionally, typical polyurethane catalysts and also other additives, an addition of 0.001 to 0.05 equivalent of a carboxylic acid per equivalent isocyanate to the reaction mixture being of particular importance, the carboxylic acid being used not as the actual blowing agent, but instead to modify the properties. According to the invention, water and/or readily volatile organic compounds are used as blowing agents. The catalysts used in the polymerization reaction are compounds which initiate a trimerization reaction at temperatures as low as room temperature, for example Mannich bases and secondary amines. The polyurethane reaction is carried out in the presence of typical catalysts such as, for example,. N-methyl morpholine, 1,4-diazabicyclo-(2,2,2)-octane,N-methyl-N'-dimethylaminoethyl piperazine and 1,2-dimethyl imidazole. The disadvantage of this process is that water or volatile organic compounds have to be used as blowing agents. Thus, trichlorofluoromethane is used as blowing agent in all the Examples.
EP 423 594 describes a process for the production of polyurethane foam moldings having a density of at least 250 kg/m.sup.3, for which purpose an aromatic polyisocyanate is reacted with an organic polyhydroxyl compound in the presence of a salt of an organic carboxylic acid with a nitrogen base containing at least one N--H bond. Amines containing tertiary amino groups may also be used providing they contain at least one primary or secondary amino group in addition to the tertiary amino group, such as N,N-dimethyl-1,3-propylene diamine for example. One of the disadvantages of this process is that, in the absence of further blowing agents, it is only possible to produce semirigid to rigid integral foams.
DE 38 40 817 describes a process for the production of polyurethane foam moldings and to the moldings obtained by this process, a density of at least 250 kg/m.sup.3 being obtained. Carboxylic acids are used as blowing agents. Carboxylic acids containing at least one other isocyanate-reactive group in addition to the carboxyl group, such as for example lactic acid and aqueous solutions thereof, are particularly preferred. Tertiary amines and organometallic compounds are used as catalysts. In this case, too, the relatively high density of the foams and the preferred mold temperature of 50.degree. C. are disadvantages.
GB 863,466 describes the production of a foam of a) a copolymer of a conjugated diene and an aliphatic unsaturated carboxylic acid containing up to 6 carbon atoms and b) an organic polyisocyanate. Water or a dicarboxylic acid is preferably added to influence the density. The reaction rate is controlled through the temperature and by the addition of bases. The following bases are specifically mentioned: diphenyl amine, p-phenylene diamine, diphenyl guanidine, guanidine, aniline, benzidine, o,o'-dichlorobenzidine, anisidine, aminopyridine, 2,2-dipyridyl amine, 2-amino-4,6-dimethyl pyridine, hexamethylene tetramine, hydrazine hydrate, calcium hydroxide and ammonium carbonate. In the Examples, the reaction temperature is in the range from 70.degree. to 110.degree. C. The reaction lasts about 1 hour. No particulars are provided as to the density of the foams obtained by this process.
Various patent specifications of Union Carbide Corporation (for example U.S. Pat. No. 4,528,334) describe carboxylated polyols produced by grafting of acrylic acid (3 to 15% by weight onto poly(oxyalkylenes). Products of this type have meanwhile been marketed under the name of UCARMOND. They have molecular weights in the range from about 400 to 3,000. Similar products are described in EP 119 349 (Olin Corporation) for the production of PUR dispersions. In this case, however, maleic acid and/or fumaric acid are used for grafting. However, the products are also used for the production of microcellular polyurethane elastomers (see Proceedings of the PURWorld Congress, September 1991, pages 686 to 690). In the application described therein, the raw materials have to be preheated to temperatures of 33.degree. C. or 40.degree. C. while the mold has to be heated to a temperature of 50.degree. C. in order to obtain an adequate reaction rate. The elastomers obtained vary in density from 160 to 320 kg/m.sup.3 according to the percentage of acids grafted on.
WO 91/00305 (Batelle Institute) relates to plastics based on fatty acids, difatty acid diamides, diesters, amidoesters, monofatty acid amidoamines or monofatty acid amidoalcohols containing at least two functional groups being used as the monomer units. The production of elastic foam plastics from 12-hydroxystearic acid and hexamethylene diisocyanate is described in Example 11. The reaction only takes place at relatively high tempratures (150.degree. C.). No particulars of the density of the foam obtained are provided.
German patent application DE 41 20 432 describes dihydroxyfatty acids suitable as a structural unit for use in polyurethane systems. The production of aqueous polyurethane dispersions is described as a potential application, non-aromatic isocyanates preferably being used and the carboxyl group not reacting with the NCO group.
To sum up, it may be said that, hitherto, the use of carboxylic acids as blowing agents for PUR systems has been attended by major disadvantages, including in particular the application of relatively high temperatures, an inadequate blowing effect, high densities, incomplete reactions, the formation of toxic and inflammable gases. These are all obstacles to industrial application, as stated in DE 30 41 589.